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INHALATION ANESTHETICS
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Currently used inhalation agents include nitrous oxide, halothane, isoflurane, desflurane, and sevoflurane. Inhalation anesthetics, notably halothane and sevoflurane, are particularly useful for the inhalation induction of pediatric patients in whom it may be difficult to start an intravenous line. Emergence depends primarily upon redistribution of the agent from the brain, followed by pulmonary elimination. Because of their unique route of administration, inhalation anesthetics have useful pharmacological properties not shared by other anesthetic agents.
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The actual composition of the inspired gas mixture depends mainly on the fresh gas flow rate, the volume of the breathing system, and any absorption by the machine or breathing circuit. The greater the fresh gas flow rate, the smaller the breathing system volume, and the lower the circuit absorption, the closer the inspired gas concentration will be to the fresh gas concentration.
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1. Factors Affecting Alveolar Concentration (FA)
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Because anesthetic agents are taken up by the pulmonary circulation during induction, alveolar concentrations lag behind inspired concentrations (FA/FI <1.0). The greater the uptake, the slower the rate of rise of the alveolar concentration and the lower the FA:FI ratio.
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The alveolar partial pressure is important because it determines the partial pressure of anesthetic in the blood and, ultimately, in the brain. Similarly, the partial pressure of the anesthetic in the brain is directly proportional to its brain tissue concentration, which determines the clinical effect. The faster the uptake of anesthetic agent, the greater the difference between inspired and alveolar concentrations and the slower the rate of induction. Three factors affect anesthetic uptake: solubility in the blood, alveolar blood flow, and the difference in partial pressure between alveolar gas and venous blood.
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The relative solubilities of an anesthetic in air, blood, and tissues are expressed as partition coefficients (Table 6–1). Each coefficient is the ratio of the concentrations of the anesthetic gas in each of two phases at steady state. The higher the blood/gas coefficient, the greater the anesthetic’s solubility and the greater its uptake by the pulmonary circulation.
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The second factor that affects uptake is alveolar blood flow, which—in the absence of pulmonary shunting—is equal to cardiac output. If the cardiac output drops to zero, so will anesthetic uptake. As cardiac ...